Evolution of radio access schemes and radio networks for cellular mobile communication (hereinafter referred to as “Long Term Evolution (LTE)” or “Evolved Universal Terrestrial Radio Access (EUTRA)”) has been standardized in the 3rd Generation Partnership Project (3GPP). In LTE, an orthogonal frequency division multiplexing (OFDM) scheme, which is a multi-carrier transmission scheme, is used as a communication scheme for wireless communication from a base station apparatus to a mobile station apparatus (hereinafter referred to as downlink (DL)). Also, in LTE, a single-carrier frequency division multiple access (SC-FDMA) scheme, which is a single-carrier transmission scheme, is used as a communication scheme for wireless communication from a mobile station apparatus to a base station apparatus (hereinafter referred to as uplink (UL)). In LTE, a discrete Fourier transform-spread OFDM (DFT-Spread OFDM) scheme is used as an SC-FDMA scheme.
LTE-A (LTE-Advanced), which is a development of LTE and in which new technology is applied, has been studied. In LTE-A, support of at least the same channel structure as LTE is being discussed. A channel means a medium used to transmit a signal. A channel used in a physical layer is referred to as a physical channel, and a channel used in a medium access control (MAC) layer is referred to as a logical channel. The types of physical channels include a physical downlink shared channel (PDSCH) used to transmit and receive downlink data and control information, a physical downlink control channel (PDCCH) used to transmit and receive downlink control information, a physical uplink shared channel (PUSCH) used to transmit and receive uplink data and control information, a physical uplink control channel (PUCCH) used to transmit and receive control information, a synchronization channel (SCH) used to establish downlink synchronization, a physical random access channel (PRACH) used to establish uplink synchronization, a physical broadcast channel (PBCH) used to transmit downlink system information, and so forth. A mobile station apparatus or a base station apparatus maps signals generated from control information, data, or the like to respective physical channels, and transmits the signals. Data transmitted on the physical downlink shared channel or the physical uplink shared channel is referred to as a transport block.
Control information mapped to the physical uplink control channel is referred as uplink control information (UCI). The uplink control information is control information (a receive confirmation acknowledgement; ACK/NACK) representing a positive acknowledgement (ACK) or a negative acknowledgement (NACK) for received data mapped to the physical downlink shared channel, control information (Scheduling Request: SR) representing a request for assignment of uplink resources, or control information (Channel Quality Indicator: CQI) representing downlink reception quality (also referred to as channel quality).
<Coordinated Communication>
In LTE-A, to reduce or suppress interference experienced by a mobile station apparatus in a cell edge area, or to increase the receive signal power, intercell coordinated communication (Cooperative Multipoint: CoMP communication) in which adjacent cells cooperatively communicate with each other is being discussed. For example, a form in which a base station apparatus communicates using an arbitrary single frequency band is referred to as a “cell”. For example, a method in which weighting signal processing (precoding processing) that differs among a plurality of cells is applied to a signal, and a plurality of base station apparatuses coordinate with each other to transmit the signal to the same mobile station apparatus (also referred to as Joint Processing or Joint Transmission) and so forth are being discussed as intercell coordinated communication. With this method, the signal power to interference plus noise power ratio of a mobile station apparatus can be enhanced, and the reception performance in the mobile station apparatus can be improved. For example, a method in which a plurality of cells coordinate with each other to perform scheduling for a mobile station apparatus (Coordinated Scheduling: CS) is being discussed as intercell coordinated communication. With this method, the signal power to interference plus noise power ratio of a mobile station apparatus can be enhanced. For example, a method in which a plurality of cells coordinate with each other to transmit a signal to a mobile station apparatus by applying beamforming (Coordinated beamforming: CB) is being discussed as intercell coordinated communication. With this method, the signal power to interference plus noise power ratio of a mobile station apparatus can be enhanced. For example, a method in which a signal is transmitted using a certain resource in only one cell and a signal is not transmitted using a certain resource in one cell (Blanking, Muting) is being discussed as intercell coordinated communication. With this method, the signal power to interference plus noise power ratio of a mobile station apparatus can be enhanced.
Regarding a plurality of cells used in coordinated communication, different cells may be configured by different base station apparatuses, different cells may be configured by different RRHs (Remote Radio Heads, an outdoor radio unit smaller than a base station apparatus; also referred to as a Remote Radio Unit: RRU) managed by the same base station apparatus, different cells may be configured by a base station apparatus and an RRH managed by the base station apparatus, and different cells may be configured by a base station apparatus and an RRH managed by another base station apparatus different from the base station apparatus.
A base station apparatus with wide coverage is generally referred to as a macro base station apparatus. A base station apparatus with narrow coverage is generally referred to as a pico base station apparatus or a femto base station apparatus. For RRHs, operation in areas of narrower coverage than of macro base station apparatuses is being discussed. A deployment such as a communication system which includes a macro base station apparatus and an RRH, and in which the coverage supported by the macro base station apparatus includes part or all of the coverage supported by the RRH, is referred to as a heterogeneous network deployment. In a communication system with such a heterogeneous network deployment, a method in which a macro base station apparatus and an RRH coordinate with each other to transmit a signal to a mobile station apparatus located in a mutually overlapping coverage area is being discussed. Here, the RRH is managed by the macro base station apparatus, and transmission/reception is controlled. The macro base station apparatus and the RRH are connected to each other by a wired link such as an optical fiber, or by a wireless link using a relay technology. In this way, the macro base station apparatus and the RRH perform coordinated communication using some or all of the same radio resources, and accordingly overall frequency utilization efficiency (transmission capacity) can be increased within the area of coverage constructed by the macro base station apparatus.
In the case of being located near a macro base station apparatus or an RRH, a mobile station apparatus is capable of performing single-cell communication with the macro base station apparatus or the RRH. In other words, a certain mobile station apparatus communicates with a macro base station apparatus or an RRH without using coordinated communication to transmit or receive a signal. For example, the macro base station apparatus receives an uplink signal from a mobile station apparatus that is close in distance to the macro base station apparatus. For example, the RRH receives an uplink signal from a mobile station apparatus that is close in distance to the RRH. Furthermore, in a case where the mobile station apparatus is located near an edge of coverage constructed by the RRH (cell edge), countermeasures against the same channel interference from the macro base station apparatus is necessary. For multi-cell communication (coordinated communication) between a macro base station apparatus and an RRH, a method for reducing or suppressing interference experienced by a mobile station apparatus in a cell edge area by using a CoMP scheme in which adjacent base station apparatuses coordinate with each other is being discussed.
Also, it is discussed that a mobile station apparatus receives signals transmitted from both a macro base station apparatus and an RRH using coordinated communication in the downlink, and transmits a signal to any one of the macro base station apparatus and the RRH in an appropriate form in the uplink. For example, the mobile station apparatus transmits an uplink signal with transmit power that is suitable for the macro base station apparatus to receive the signal. For example, the mobile station apparatus transmits an uplink signal with transmit power that is suitable for the RRH to receive the signal. Accordingly, unnecessary interference in the uplink can be reduced, and the frequency utilization efficiency can be enhanced.
It is necessary for a mobile station apparatus to obtain, regarding a process of receiving a data signal, control information representing a modulation scheme to be used for the data signal, a coding rate, a spatial multiplexing number, a transmit power adjustment value, assignment of resources, and so forth. In LTE-A, introduction of a new control channel for transmitting control information regarding a data signal is being discussed (NPL 1). For example, improvement of the capacity of the entire control channel is being discussed. For example, support of interference coordination in the frequency domain for a new control channel is being discussed. For example, support of spatial multiplexing for a new control channel is being discussed. For example, support of beamforming for a new control channel is being discussed. For example, support of diversity for a new control channel is being discussed. For example, use of a new control channel in a new type of carrier is being discussed. For example, not transmitting a reference signal common to all mobile station apparatuses in a cell in a new type of carrier is being discussed. For example, reducing the frequency of transmitting a reference signal common to all mobile station apparatuses in a cell in a new type of carrier to be lower than in a conventional technology is being discussed. For example, demodulating a signal such as control information using a reference signal specific to a mobile station apparatus in a new type of carrier is being discussed.
For example, applying coordinated communication and multi-antenna transmission for a new control channel as application of beamforming is being discussed. Specifically, it is discussed that a plurality of base station apparatuses and a plurality of RRHs compatible with LTE-A apply precoding processing for a signal on a new control channel, and the same precoding processing is applied to a reference signal (RS) for demodulating the signal on the new control channel. Specifically, it is discussed that a plurality of base station apparatuses and a plurality of RRHs compatible with LTE-A map, in the region of resources where a PDSCH is mapped in LTE, a signal on a new control channel and an RS to which the same precoding processing is applied, and transmit the signal and RS. It is discussed that a mobile station apparatus compatible with LTE-A demodulates the signal on the new control channel subjected to precoding processing using the RS that has been received and subjected to the same precoding processing, and obtains control information. With this method, it is not necessary to transmit and receive information regarding the precoding processing applied to a signal on a new control channel between a base station apparatus and a mobile station apparatus.
For example, as application of diversity, a method for obtaining an effect of frequency diversity by forming a signal on a new control channel using resources separated in the frequency domain is being discussed. On the other hand, a method for forming a signal on a new control channel using resources not separated in the frequency domain in a case where beamforming is applied to the new control channel is being discussed.
For example, as support of spatial multiplexing, application of MU-MIMO (Multi User-Multi Input Multi Output) in which control channels for different mobile station apparatuses are multiplexed on the same resource is being discussed. Specifically, it is discussed that a base station apparatus transmits reference signals that are orthogonal to each other between different mobile station apparatuses and transmits signals on different new control channels by spatially multiplexing the signals on a common resource. For example, spatial multiplexing of signals on different new control channels is realized by applying appropriate beamforming (precoding processing) to each of the signals on the different new control channels.